1. Field of the Invention
The present invention relates to a solution for automatically aligning one or more arrays of printing elements in a printing apparatus. More specifically the present invention is related to the automatic alignment of ink jet print heads in an ink jet printing system.
2. Description of the Related Art
Inkjet printing is a non-impact method for producing images by the deposition of ink droplets in a pixel-by-pixel manner into an image-recording element in response to digital signals. There are various methods which may be utilized to control the deposition of ink droplets on the receiver member to yield the desired image. In one process, known as drop-on-demand inkjet printing, individual droplets are ejected as needed on to the recording medium to form the desired image. Common methods of controlling the ejection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation using heated actuators. With regard to heated actuators, a heater placed at a convenient location within the nozzle or at the nozzle opening heats ink in selected nozzles and causes a drop to be ejected to the recording medium in those nozzles selected in accordance with image data. With respect to piezoelectric actuators, piezoelectric material is used in conjunction with each nozzle and this material possesses the property such that an electrical field when applied thereto induces mechanical stresses therein causing a drop to be selectively ejected from the nozzle selected for actuation. The image data provided as signals to the print head determines which of the nozzles are to be selected for ejection of a respective drop from each nozzle at a particular pixel location on a receiver sheet.
In another process known as continuous inkjet printing, a continuous stream of droplets is discharged from each nozzle and deflected in an imagewise controlled manner onto respective pixel locations on the surface of the recording member, while some droplets are selectively caught and prevented from reaching the recording member. Inkjet printers have found broad applications across markets ranging from desktop document and pictorial imaging to short run printing and industrial labeling.
A typical inkjet printer reproduces an image by ejecting small drops of ink from the print head containing an array of spaced apart nozzles, and the ink drops land on a receiver medium (typically paper, coated paper, etc.) at selected pixel locations to form round ink dots. Normally, the drops are deposited with their respective dot centers on a grid or raster, with fixed spacing in the horizontal and vertical directions between grid or raster points. The inkjet printer may have the capability to either produce only dots of the same size or of variable size. Inkjet printers with the latter capability are referred to as (multitone) or gray scale inkjet printers because they can produce multiple density tones at each selected pixel location on the page.
Inkjet printers may also be distinguished as being either pagewidth printers or swath (scanning) printers. Pagewidth printers are equipped with a pagewidth print head or print head assembly capable of printing one line at a time across the full width of a page. The line is printed as a whole as the page moves past the pagewidth print head while the print head is stationary. Pagewidth printers are also referred to as single pass printers because the image area is printed in only one pass of the page past the print head. An example of a pagewidth printer is the :Dotrix Modular printer commercially available from Agfa Graphics NV (Belgium).
Swath printers on the other hand use multiple passes to print an image. In each pass a swath of the image is printed on the page. The width of a swath typically is linked to the print width of the print head or print head assembly used for printing the swath while passing across the page. Between such passes the page is advanced relative to a position of the print head so that a next pass of the print head across the page prints a next swath of the image next to or (partially) overlapping the already printed swath. In swath printers a print head is traversed in a fast scan direction during a pass across the page to be printed. Often the traversal is perpendicular to the direction of the arrangement of the array of nozzles of the print head. The page to be printed moves in a slow scan direction, typically perpendicular to the fast-scan direction. An example of a swath printer is the :Anapurna large format printer commercially available from Agfa Graphics NV (Belgium).
Print heads or print head assemblies used in both pagewidth printers and swath printers may include multiple arrays of nozzles mounted together as a single module in a print head or print head assembly. The arrays may be arranged in an interleaved position along the fast scan direction to increase print resolution or may be arranged to abut each other to increase the print (swath) width of the print head. The arrays may be arranged after each other with their respective nozzles in line with each other along the print direction. The first types of arrangements are often used to create improved monochrome print head assemblies, whereas the latter arrangement is often used in the design of multicolor print head assemblies.
To create pleasing printed images, the dots printed by one nozzle array must be aligned such that they are closely registered relative to the dots printed by the other nozzle arrays. If they are not well registered, then the maximum density attainable by the printer will be compromised, banding artifacts will appear and inferior color registration will lead to blurry or noisy images and overall loss of detail. These problems make good registration and alignment of all the nozzle arrays within an inkjet printer critical to ensure good image quality. That is, not only should a nozzle array be well registered with another that jets the same color ink, but it should be well registered with nozzle arrays that jet ink of other colors.
In addition to good image quality, faster print rates are desired by customers of inkjet printers. For swath printers, a well-known means by which to accomplish high productivity is to increase the number of nozzles. One way in which nozzle count may be increased is by simply adding extra nozzle arrays. This has the advantage that the same print head design may be used. However, this adds to the number of nozzle arrays that must be aligned, thereby increasing the possibility for misalignment and the labor required to properly align all the nozzle arrays.
An alternative to gain higher productivity is to increase the nozzle count within a nozzle array. This does not increase the count of nozzle arrays, but usually results in longer nozzle arrays as increasing the nozzle density of a nozzle array typically requires a completely new print head design and/or a new manufacturing process. Longer nozzle arrays also increase the difficulty of alignment of the nozzle arrays as the sensitivity to angular displacements increases proportionately.
In high-end inkjet printers, such as one that might be used in a wide-format application, there are still other considerations that must be made to ensure proper alignment of the nozzle arrays. For instance, bi-directional printing in the fast-scan direction to increase productivity requires that the nozzle arrays be properly aligned whether traveling in the right-to-left direction or the left-to-right direction.
Some high-end printers accept a variety of ink- receiving materials that may differ significantly in thickness. As a result, the printer may have several allowable discrete gaps between the nozzle arrays and the printer platen to accommodate these different receivers. Invariably, the gap between the nozzle arrays and the top of the receiver, referred to as the throw-distance, can vary significantly because of the range of receiver thicknesses and the limited number of discrete nozzle array heights. Due to the carriage velocity, the flight path of the drop is not straight down but really is the vector sum of the drop velocity and carriage velocity. This angular path and the differences in throw-distance make nozzle array registration sensitive to both the average of throw-distance as well as the variation in the throw-distance. These sensitivities further complicate the nozzle array alignment process.
Additionally, some high-end printers allow the customer to select different carriage velocities, with higher carriage velocities resulting in increased productivity usually at the expense of image quality. The term “carriage velocities” implies the supporting of the print heads upon a carriage support that moves in the fast-scan direction while being supported for movement by a rail or other support. The angular flight path of the droplets described will be a function of the carriage velocity. This then makes nozzle array alignment sensitive to yet another variable, namely carriage velocity.
Current alignment techniques fall within two varieties. Visual techniques use patterns printed by the printer that permit a user to simultaneously view various alignment settings and choose the best setting. Visual techniques are disadvantaged in many ways. Firstly, for a printer with many nozzle arrays (twenty-four separate nozzle arrays is not uncommon), multiple throw-distances, and multiple carriage velocities, the number of alignments can become overbearing as each variation adds multiplicatively to the rest. Secondly, only a moderate level of accuracy is attainable with most of these techniques and finely tuned printers require a higher degree of accuracy than is attainable by most of these techniques. Thirdly, interactions can occur between the various alignment parameters, which further degrade the ultimate quality of alignment that can be obtained through these visual techniques, or multiple iterations are required, thereby increasing the labor of the effort. Lastly, since several of these techniques usually operate by providing several alignment settings to the operator who then chooses the best choice, significant amounts of consumables (ink and media) may be required to obtain satisfactory alignment of all nozzle arrays in all print modes.
The second way nozzle arrays are typically aligned is with an on-carriage optical sensor that interprets patterns printed by the nozzle arrays to automatically make adjustments to the nozzle array alignment. While much improved over the more common visual techniques, these methods, too, have several shortcomings. Firstly, the optical sensors are typically of the LED variety with economical optics and cannot provide the high degree of accuracy required of finely tuned, high-end printers. Secondly, these sensors require significant averaging to create a reliable signal, making the amount of receiver required to perform the alignment larger than one would desire. Furthermore, this high degree of averaging necessitates a separate measurement for each nozzle array, requiring even more ink and receiver as the number of nozzle arrays increases. Thirdly, these on-carriage optical sensors are typically arranged to provide data primarily in the fast-scan direction. For demanding applications, slow-scan adjustments are equally important. Some techniques provide means by which slow-scan misalignments may be determined, but these measurements require separate, additional patterns, further consuming additional ink and receiver. Furthermore, this fast-scan limitation makes determination of nozzle array skew very difficult or impossible. Another result of the fast-scan directional limitation is the inability to measure errors in the movement of the receiver, yet another critical alignment variable.
It is therefore desired to develop a nozzle array alignment technique and process that provides a high degree of accuracy of alignment of all critical alignment variables while requiring very little labor and time to execute and consuming as little ink and receiver as possible.